1. Field of the Invention
This invention relates to an electrostatic copying machine which uses a photosensitive screen having a number of fine openings therein.
2. Description of the Prior Art
As the typical conventional electrophotography, a direct process such as for example electrofax and an indirect process such as xerography are presented. In the former direct process, use is made of a specifically treated recording material coated with a photoconductive material such as zinc oxide. Consequently, there is a drawback in the image contrast as the image formed on the recording material lacks brightness. Moreover, due to the specific treatment, the recording material is heavier than the conventional paper and has a different feed from the usual paper. According to the latter indirect process, a high contrast and high quality image is obtained since it uses plain paper as the recording material to form an image. However, in this indirect process, when a toner image is transferred to the recording material, the recording material contacts with the surface of the photosensitive member and further, cleaning means strongly contacts with the surface of the photosensitive member when the remaining toner is removed, so that the photosensitive member undergoes a deteriorating each time the transfer and the cleaning is effected. Therefore, the durability of an expensive photosensitive member is reduced and this results in a high cost for the image formation.
The improvements for eliminating the above-described drawbacks of the conventional process were disclosed in, for example, U.S. Pat. Nos. 3,220,324; 3,680,954 and 3,645,614. In these patents, a photosensitive member of the screen type or the grid type is used which has a number of fine mesh-like openings. The electrostatic latent image is formed on a recording material by modifying ion flow through the screen or grid, and thereafter the recording material with the latent image formed thereon is visualized. There is no necessity of developing and cleaning the screen or grid which corresponds to the photosensitive member, so that the durability of the screen or grid is increased.
In U.S. Pat. No. 3,220,324, a conductive screen coated with a photoconductive material is used and corona ions from a corona discharger and an image exposure applied simultaneously to a recording material through said screen. The corona ion flow is modified by the screen and an electrostatic latent image is formed on the recording member.
According to U.S. Pat. No. 3,680,954, a conductive grid coated with a photoconductive material and a conductive controlling grid are used, an electrostatic latent image is formed on the grid in the image form, and different electric fields are formed on the grid and the controlling grid so as to modify the corona ion flow to form an image on a recording member.
In U.S. Pat. No. 3,645,614, the screen comprises an insulating material overlaid with a conductive material and the insulating material comprises a photoconductive material. An electric field preventing the passage of ion flow is formed at the openings for passing the ion flow due to the electrostatic latent image formed on the screen.
In an image forming apparatus using the above-described screen having a number of fine openings therein, a sheet-like recording material such as insulating paper or the like on which an image is to be formed by modulated ions must be positioned closely adjacent the primary electrostatic latent image on the screen in order to provide a good copy image. For this purpose, a device has heretofore been considered which moves a flat or a drum-shaped screen relative to a recording material being conveyed by a stationary conveyor means so that the primary electrostatic latent image on the screen may be positioned closely adjacent the recording material. However, movement of the screen has raised difficulties in obtaining a mechanical accuracy of the device and also led to a larger size of the device.